1. Title of the Invention:
The present invention relates to a screw vacuum pump having a pair of screw rotors meshed with each other and, more particularly, to an oilless screw pump suitable for application to industries such as the semiconductor industries where air containing foreign matters is detrimental to the products and for application to the industry where odorous air must be avoided.
2. Description of the Prior Art:
Various vacuum pumps including screw vacuum pumps for such applications have been proposed. For example, Japanese Patent Publication No. 54-37693 discloses a water-sealed vacuum pump, Japanese Patent Publication No. 57-59920 discloses a high-vacuum pump, and Japanese Patent Provisional Publication (Kokai) No. 59-185889 discloses a dual shaft vacuum pump.
The oilless screw vacuum pump is one of the desirable pumps. Basically, the screw vacuum pump is of the same construction as the compressor. However, since the screw vacuum pump is used in a situation where the pressure of the suction gas is very low, the compression ratio of the screw vacuum pump, namely, the ratio of the pressure of the discharge gas to that of the suction gas, is very high. For example, the discharge pressure of the vacuum pump, in general, is atmospheric pressure (1.033 ata), while the suction pressure is on the order of 0.1 ata or 0.01 ata. Consequently, the temperature of the discharge gas is elevated to a very high level due to adiabatic compression of the gas. Discharge gas temperature Td (.degree.K) is expressed by EQU Td=Ts (P2/P1).sup.(.kappa.-1)/.kappa.
where P1 is suction pressure, P2 is discharge pressure, Ts (.degree.K) is suction gas temperature, and .kappa. (1.4 for air) is the specific heat of the gas. Accordingly, in the vacuum pump, the compression ratio P2/P1 is very large when the suction pressure is on the order of 0.01 or 0.001 torr, and hence the discharge gas temperature Td is elevated to a very high level. For example, in such air (.kappa.=1.4) of an ordinary temperature of 30.degree. C. (303.degree. K.) by the vacuum pump, the discharge gas temperature Td is 317.degree. C. (590.degree. K.) and 867.degree. C. (1140.degree. K.) when the suction pressure P1 is 0.1 ata (76 torr) and 0.01 ata (7.6 torr), respectively. Thus, the vacuum pump exerts a relatively small amount of work on the gas, but elevates the temperature of the gas to a considerably high level.
To suppress a rise in temperature of the gas, the water-sealed vacuum pump or the high-vacuum pump, the gas is brought into direct contact with water or oil while the gas flows from the suction port to the discharge port so that the gas and the vacuum pump is cooled. However, such vacuum pump suffers from the leakage of water or oil in the suction side and the reverse flow of water or oil in case the vacuum pump is stopped suddenly or when the power supply is interrupted accidentally. Accordingly, such vacuum pump is inapplicable to the semiconductor industries and the food industries for the above-mentioned reasons.
There has been proposed an oilless dual shaft vacuum pump which brings the gas in contact with neither water nor oil. In this specification, "oilless" means not requiring oil or water which is to be brought into contact with the gas while the gas flows through the vacuum pump. The oilless dual shaft vacuum pump is applicable for operation at a vacuum in a medium vacuum range. However, since the rotors of the oilless dual shaft vacuum pump must be positioned with a gap therebetween, the leakage of the gas through the gap between the rotors increases when the oilless dual shaft vacuum pump is applied for operation in a low vacuum range.